Adjoining concrete slabs in highways, aircraft runways, warehouses, or the like, are repeatedly subjected to heavy wheel-borne loads which tend to deflect downwardly first one and then the other of the slabs as the wheel passes over the joint separating the two. As a result, vertical relative motion occurs along the slip plane of the joint.
One consequence of this relative motion is the "pumping" action produced by the slab as it first tilts up and then down. If the ground underlying the slab becomes muddy, the pumping action forces mud upwardly through openings in the joint, thereby depleting the material of the supporting ground surface and, in many cases, causing the appearance of voids, or cavities, under the slab. It is then only a matter of time until the repeated unsupported stresses cause small fractures to appear, followed by more serious ruptures which eventually compel major repairs, or even replacement, accompanied by costly and inconvenient down-time.
Numerous attempts have been made to circumvent the problem. For the most part, the efforts have been in the direction of providing a strong physical link, such as a dowel, embedded in the concrete in such a way as to span both of the adjoining slabs. Dowels are generally installed at the time of construction, as in Older U.S. Pat. No. 2,194,718 for Concrete Road Joint, and if properly installed they perform well.
If adequate load transfer is not provided for in the joints at the time of construction, it has heretofore been difficult, if not impossible, to install adequate load transfer in existing slab joints. The generally accepted method of installing load transfer devices in existing concrete slab joints has been to cut slots or trenches in the existing concrete across the joint and to install dowels in these slots and cement them in place.
This method of dowel installation in finish concrete can create inordinate stress concentrations in existing concrete slabs which can seriously damage the slabs and is expensive if the dowels are installed properly. If the dowels are not installed properly, then the condition created is worse than if the dowels were never installed.
Where slab rehabilitation is required, the cost and effort of excavating a hole large enough to accommodate a device such as that disclosed in the patent to Older would be economically unfeasible, particularly where the rehabilitation of major sections of highway or large aircraft runways or warehouses, for example, entails the use of hundreds of such devices.
Furthermore, as indicated above, the excavation in existing slabs required to install these devices damages the slabs, causing stress concentrations which can lead to extensive slab damage and ultimately to complete failure.
The load transfer device embodying the present invention, on the other hand, is easy to install and does not create stress concentrations in the existing slabs since it is placed in a round hole and adequately transfers load from one slab to another.